What is multiplication?

Date: 11/09/97 at 18:32:10
From: JMTI
Subject: What is multiplication?

What does multiplication have to do with addition, geometry, and real 
life?

Date: 01/14/98 at 12:35:55
From: Doctor Sonya
Subject: Re: What is multiplication?

I hope you've figured out what multiplication has to do with all 
these things by now, but in case you haven't, here's what I think.

First, what does multiplication have to do with addition?  What 
happens when you do something like multiply 3 times 4?  (I'll write 
this as 3 * 4.)  Well, 3 * 4 = 12, right?  But ....... (drum-roll 
please)

  3 + 3 + 3 + 3 = 12 too.  

So multiplying 3 by four is like adding 3 to itself 4 times. 

I think this is why we say "3 times 4", because you are taking 3 and 
adding it 4 times.  

What about 4 * 3?  This means that we add 4 to itself 3 times:

    4 + 4 + 4 = 12

which is what we should have gotten in the first place!  Yay.

In geometry, one thing you can use multiplication for is to calculate 
areas. For example, if you have a rectangle with one side that's 
3 inches and the other that's 2 inches, what's the area? You find 
the area by multiplying the lengths of the two sides together.  

Finally, what does multiplication have to do with real life?  Here are 
some of the areas where I use multiplication.

  1. I multiply when I cook and have to double the recipe.

  2. I multiply when I try to figure out how much interest I have 
     earned on my savings account.

  3. I multiply when I calculate how many cans of frozen lemonade to 
     buy each week. (My family drinks about 1.5 cans a day, so for 
     7 days I need 1.5 * 7 = 10.5 cans. But what am I supposed to do 
     with the extra half?)

  4. Richard, who works in the Math Forum office here, says that he 
     used multiplication recently when he wanted to find out how many 
     bonds he needed to buy if he needed to make $1500 with interest.

  5. Ken, also in the office, used multiplication to calculate how 
     much money he'll make this year designing Web pages if he makes 
     $200 per page.

I hope this helps.  Write us back if you have any more questions. 

-Doctor Sonya,  The Math Forum
 Check out our web site!  http://mathforum.org/dr.math/